The present invention relates to a color cathode ray tube (CRT) display, in particular it relates to a color CRT display which has a one-beam electron gun and reproduces colors by switching color signals, R (Red), G (Green), and B (Blue), in regular succession which are supplied to the onebeam electron gun.
As a color CRT display, one which has three-beam electron guns which emit 3 electron beams for energizing respective phosphors, R, G, and B, and uses a shadow mask as a color selection electrode, is widely known. Similarly, the CRT display of the Trinitron system, having three-beam electron guns and using an aperture grille as a color selection electrode also is widely known. The two points as described in the following can be cited as common defects to these three-gun color CRT displays.
(1) Color misregistration can occur by being influenced by an external magnetic field. The influence of a static magnetic field such, as the earthxe2x80x99s magnetism, can be prevented by using a magnetic shield and a degauss coil. However, when a CRT is used in a fluctuating magnetic field such as the field close to a railway, it is apt to be influenced by an external field. Further, when there are other electronic appliances or magnets close to the display, the color misregistration can be caused by them.
(2) Convergence adjustment is required for converging the three electron beams corresponding to R, G, and B emitted from the three guns at a point on a color selection electrode (shadow mask, aperture grille). In particular, in the case of a high precision display corresponding to a graphics display standard such as a UXGA (ultra extended graphics array), high precision convergence correction is required.
There is a color CRT display of a beam-index system which is contrived to solve the problems with a color CRT display of the shadow mask system or the Trinitron system.
The color CRT display of the beam-index system has a one-beam electron gun and is constituted with a fluorescent screen having phosphor stripes, R, G, and B, and carbon (black) stripes between respective phosphor stripes.
When the fluorescent screen is scanned by an electron beam emitted from the one-beam electron gun, the chrominance signals input to the electron gun are switched corresponding to the color of a phosphor stripe irradiated by the electron beam, thus respective colors are displayed. An index stripe separately provided on the fluorescent screen is used for the switching of the chrominance signals.
When the color CRT display of the beam-index system having a constitution as mentioned in the above is applied to a high precision display corresponding to a UXGA display standard, the number of the phosphor stripes of R, G, and B is increased. Therefore, it is necessary to increase the number of index stripes corresponding to the degree of preciseness. When the number of index stripes is increased, the index frequency becomes higher.
However, a phosphor for index stripes having enough short afterglow time does not exist at present. Thereby, when the index frequency becomes high, the detectable index signal quantity is decreased. In such a case, a black level beam current that is made to flow as a minimum beam current becomes large in order to obtain a stable index signal even at a black video signal. Therefore, a black compression is apt to be generated and, in this case, the contrast is substantially degraded.
It is an object of the present invention to provide a beam-index system color CRT display of a one-beam electron gun which is able to perform stable color reproduction even in a case where it is applied to a high precision display without generating a black compression.
A color CRT display, according to the present invention, comprises a one-beam electron gun which emits an electron beam and a fluorescent screen on which phosphor stripes, which emit R, G, and B rays of light when irradiated by the electron beam, are disposed in regular succession in the main scanning direction of the electron beam, and wherein the chrominance signals, R, G, and B, to be supplied to the one-beam electron gun, are switched over to each other in regular succession for the reproduction of colors.
The color CRT display comprises an image pickup device for picking up a color image displayed on the screen of the CRT, a first storage means for storing the image pickup signal outputted from the image pickup device, a second storage means for storing an input video signal, a switching means for switching over respective chrominance signals, R, G, and B, of the input video signals to be supplied to the one-beam electron gun, a variable frequency oscillator means for outputting an oscillation frequency signal which becomes the base of switching timing of chrominance signals by the switching means, a phase modulation means for phase-modulating the oscillation signal output from the variable frequency oscillator means and. for supplying the modulated signal to the switching means as a switching control signal, and a control means which compares the component of chromaticity of the image pickup signal stored in the first storage means with the component of chromaticity of the video signal stored in the second storage means and controls the oscillation frequency of the variable frequency oscillator means and the phase modulation by the phase modulation means to make the difference obtained by the comparison minimum.
In a color CRT display having the configuration mentioned above, when an electron beam scans the phosphor stripes, R, G, and B, on the fluorescent screen in the main scanning direction in order, the respective chrominance signals, R, G, and B, to be supplied to the one-beam electron gun is switched over to each other in regular succession by the switching means. Thus, a color image is displayed on the CRT screen, and the color image is picked up by an image pickup device. The image pickup signal is stored in the first storage means.
On the other hand, the video signal being the base of the color image is stored in the second storage means. The control means compares respective components of chromaticity between the image pickup signal and the video signal stored in respective storage means and controls the oscillator frequency and the phase modulation which are the base of the switching timing of respective chrominance signals, R, G, and B, to minimize the result of the above comparison.
Thereby, even in the case where the present invention is applied to a high-precision display, a color misregistration can be prevented by the above mentioned feedback control, so that the color reproduction can be stably performed without generating a color misregistration or a black compression caused by an external magnetic field.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.